The present invention relates to a magnetic resonance imaging apparatus and an A-D conversion device wherein an NMR signal acquired by scanning an object to be examined is subjected to analog-digital conversion.
In recent years, MRI (Magnetic Resonance Imaging) apparatus utilizing a magnetic resonance phenomenon has been in wide spread use, and has been extensively used as a technique indispensable to diagnostic imaging of organs in living bodies.
The RF receiver unit (A-D conversion unit) of an MRI apparatus receives an NMR signal (Nuclear Magnetic Resonance signal) from an object to be examined, subjects it to A-D conversion, and outputs the obtained signal to a computer (signal processing unit). As described in Patent Document 1, for example, such an RF receiver unit is often constructed separately from a signal processing unit so that it can be expanded so as to enable the reception of multichannel incoming data.
[Patent Document 1] Japanese Unexamined Patent Publication No. 2002-353812
An MRI apparatus is constructed of: a scan unit that receives an analog NMR (Nuclear Magnetic Resonance) signal from an object to be examined; and a signal processing unit that carries out signal processing. Since the scan unit is placed in a high-magnetic-field environment, the scan unit and the signal processing unit are often installed separately from each other from the viewpoint of protection of the signal processing unit against a magnetic field and other like viewpoints. For example, they are often placed in different rooms.
An A-D (Analog-Digital) conversion device is provided which converts an analog NMR signal received by the scan unit into a digital signal, and a converted digital signal is transmitted to a signal reception unit. Receiving digital signal, the signal processing unit carries out signal processing, such as digital filtering, and then composes an image.
The A-D conversion device includes a portion (referred to as input unit) that is inputted with a signal from the scan unit and carries out A-D conversion and a portion (referred to as output unit) that outputs a digital signal to the signal processing unit. These portions are often disposed separately from each other, for example, placed on different boards. This is because in cases where they are disposed on one board, an analog signal and a digital signal coexist on the one board and this increases noise.
The input unit and the output unit are connected with each other through a bus for digital signals. As mentioned above, the input unit and the output unit are often installed away from each other, and the bus as the wiring for signals is inevitably longer than in cases where devices are connected together within one board. This brings the following disadvantage: noise may be produced depending on how a digital signal fluctuates on the bus, and this noise can get mixed in an analog signal and have great influence on the quality of an image. Especially, when the absolute value of an original NMR signal is small, noise caused by the fluctuation of digital signal poses a problem. Images generated by the signal processing unit are greatly influenced by the noise.